Ive been recently reading comments from other Yoto users who copy their existing Yoto Cards to MYO Cards due to wanting backups... but another common reason is incase they lose or have their card stolen. If the original card gets into the hands of someone else with a Yoto the card seems to transfer ownership to the new card holders account.
As a suggestion, why can't cards be locked to their purchases / owner, until such time as the owner wishes to unlock them. Unlocking a card may be for resale, no longer using, library getting too large, etc. Supposedly Tonies do this already.